Being affected by many severe events involving oil crisis and air pollution in recent years, it has become an inevitable trend for all kinds of traffic and transportation means to use energy-saving and environment-friendly parts. For internal combustion engine vehicles, only a small part of the power output by the internal combustion engine thereof is use to run the vehicles while other part is offset by various kinds of resistance. One example of such resistance comes from the rough road surfaces. In the past, a shock absorbing damper is used to absorb vibration and convert the vibration energy into heat energy, which is then dissipated into ambient air. Now, many related technical fields have developed the idea of recycling various kinetic energy and heat energy, such as the brake energy, damping energy and so on, by converting them into electric energy. To recycle the damping energy, a hydraulic motor is used to convert the hydraulic energy of the damping device or shock absorber into mechanical energy, which can be then used as a power source to operate a generator, to charge a battery or for other purposes, so that the load of the vehicle engine for rotating the main generator can be reduced.
The hydraulic motor for converting the hydraulic energy into mechanical energy is an element widely used in many hydraulic systems. There are many differently designed hydraulic motors, such as plunger, screw-rod, gear and vane type hydraulic motors, which have different features and are suitable for different occasions. Except the vane type hydraulic motor, the screw-rod and the gear type hydraulic motor must have an input and an output rotary shaft arranged in pairs. Therefore, the output shaft is not concentric with the case of the hydraulic motor. The plunger type hydraulic motor includes a plurality of circumferentially arranged plunger pumps and therefore has a considerably large diameter. The shock absorbers for motor vehicles are usually a long tube in configuration with a relatively small inner diameter. Therefore, to substitute a hydraulic motor for the damping valve in the vehicle shock absorber to recycle the vibration energy, it would be better to use the vane-type hydraulic motor, which is more suitable for the limited internal space of the conventional vehicle shock absorber and has a shaft coaxial with the shock absorber for connecting to a generator to output the converted mechanical energy. However, the typical vane-type hydraulic motor is enclosed in a complicated cast case with the shaft exposed from the case, and has radially oriented hydraulic fluid inlets and outlets and a fixing plate provided on one side thereof. Further, the output shaft and the rotor must be provided as two separated parts to enable assembling of the vane-type hydraulic motor. The fully assembled vane-type hydraulic motor still has an overall length and an outer diameter much larger than those of the damping valve to be replaced and therefore could not be fitted into but only be hooked to an outer side of the tubular body of the shock absorber. That is, the use of the conventional vane-type hydraulic motor to replace the damping valve would occupy an additional space in the vehicle and could not be directly used to replace the conventional tube-shaped shock absorber.